This invention relates to telecommunication switching systems and more particularly to apparatus for timing, commonly for billing purposes, of calls originated by stations served by such systems.
Telephone switching systems which control the interconnection of calling stations with called stations are usually provided with automatic message accounting equipment for assessing message charges against a calling station for billing purposes. When a call to a called subscriber originates at a called subscriber station, the switching system serving the calling station accesses message accounting equipment to record an initial entry identifying the calling station, the called station, and the switching system trunk equipment employed to establish a call connection. After the call connection has been established, the switching system trunk equipment detects the answer of the call by the called telephone station and again accesses the automatic message accounting equipment in order to record an answer entry which fixes the time the call was answered. Subsequently, the switching system trunk equipment detects the disconnect of the calling and called stations from the call connection and again accesses the automatic message accounting equipment in order that a disconnect entry fixing the disconnect time of the call connection may be recorded. Three entries are thus recorded for each telephone call by this type of automatic message accounting equipment: initial, answer, and disconnect.
The trunk equipment of a telephone switching system typically comprises a number of trunks arranged to connect the switching system with other switching systems to establish call connections between calling and called subscriber stations. Typically, the trunks are selectively connectable to a recorder which accesses an associated perforator to record the initial, answer, and disconnect entries as perforations on paper tape. The paper tapes thus produced by the perforators at various geographically dispersed central offices are transported to a centralized data processing center where the billing charges are computed. In a more recent telephone system, billing data is assembled directly from a central office recorder and transmitted via a data link to a distant automatic message accounting recording center (AMARC) where the billing data is recorded on magnetic tape. This billing data transmission arrangement advantageously overcomes problems such as the physical bulk and low information storage density, for example, encountered with the use of paper tape.
A billing data transmitter of the character contemplated is physically located at a central office, as mentioned, remote from the AMARC and is arranged to connect with central office recorder equipment. When an automatic message accounting trunk detects a call answer or disconnect, the recorder equipment registers the identity of the trunk and requests the services of the billing data transmitter (BDT). The latter records the trunk identity data togetherwith data representing the fact of an answer or disconnect. This data is stored in a BDT buffer memory to wait transmission to the distant AMARC. Since the rate of storing data in the buffer may occasionally exceed the transmission rate, a varying transmission delay is introduced between a BDT and the service AMARC. As a result, the times of receipt of call answer and disconnect data at the AMARC cannot be relied on as actual times of occurrence without some means for determining the transmission delay introduced. Since a calling subscriber is billed for the call period between answer and disconnect, high precision in recording the initiation and termination of a call is required to avoid errors in call charging. One solution to the timing problem is to provide a highly accurate clock at each BDT location to note the actual times of the answer and disconnect occurrences. Data representing these times may then be relayed to the AMARC for recording. A mechanism would be needed for setting each BDT clock and possibly also a mechanism for synchronizing each with the AMARC clock. Obviously, such a timing arrangement is both complex and costly.
An object of this invention is the precise timing of telephone call answer and disconnect occurrences in an automatic message accounting system in which variable data transmission delays may be encountered.
Another object of this invention is the provision of apparatus for accurately timing telephone call information at a central recording center, which information originates at a plurality of remotely located offices and is subject to variable transmission delays.
A further and more general object of this invention is the provision of apparatus for centrally recording the precise time of occurrence of events at a plurality of remote locations when the transmission of information regarding the events is subject to unpredictable delays.
It is also an object of this invention to simplify and reduce the cost of telephone system apparatus for accurately determining delays in the transmission to a recording center of data relating to call answer and disconnect occurrences at a remote central office.